1. Field of the Invention
This invention relates to an outboard motor air intake system.
2. Description of the Related Art
Outboard motor air intake systems have recently been proposed that use an actuator to open and close the throttle valve of the internal combustion engine incorporated in the outboard motor. Generally in a system of this type, the operator inputs an instruction to open the throttle valve by manipulating a remote control lever installed outside the outboard motor (at an appropriate location on the hull of the boat driven by the outboard motor) and the throttle opening is regulated accordingly by controlling the operation of the actuator, as taught, for example, in Japanese Patent No. 2817738, paragraphs 0013 to 0017 and FIG. 1.
However, the prior art system does not take into account an important aspect of the relationship between the throttle opening and the amount of air drawn in.
Namely, it ignores the fact that the change in amount of intake air with respect to change in throttle opening increases with decreasing throttle valve opening. This relationship arises because the pressure difference between upstream and downstream of the throttle valve increases with decreasing throttle opening. Therefore, owing to the limited resolution power of the sensor that detects the amount of remote control lever manipulation, the operator's weak sense of manipulation because of the mechanical isolation of the remote control lever from the throttle valve and various other factors, the prior art system that opens and closes the throttle valve by operating an actuator in response to the manipulation of the remote control lever makes it difficult during small throttle valve opening (including the full-closed condition) to finely regulate the amount of intake air, i.e., to finely regulate engine speed when the engine is operating in a low-speed region including the idling region.